Through much of southwestern Pennsylvania, adjacent Maryland, and West Virginia, Upper Mississippian (Chesterian) strata are represented by intertonguing of red clastics of the Mauch Chunk Formation and carbonates of the Loyalhanna and Wymps Gap Limestones.

The lower clastic wedge of the Mauch Chunk represents an episodic shoreline progradational event that buried the underlying Loyalhanna carbonates. The sea level rise concurrent with the Wymps Gap transgression resulted in nearly continuous deposition of clastics in nearshore areas while carbonates were being deposited on more offshore areas. This resulted in the development of a lobe of clastics (forming a small platform) that created significant topography to be transgressed by the deepening Wymps Gap sea. Facies development of the Wymps Gap carbonates was markedly influenced by this inherited topography. In areas where clastics are thin, the Wymps Gap is represented by a medium-bedded, dark-gray, petroliferous, clay-rich carbonate mudstone to wackestone. These sediments are representative of open shelf deposition, in moderate water depths below storm wave base. Along the margin of the thick clastic lobe, the Wymps Gap is represented by a medium-bedded, dark-gray, locally cross-bedded carbonate grainstone to packstone. These accumulations appear to represent a slope-break platform-edge shoal environment. Over the top of the thick lobe of elastics, highly argillaceous, nodular-bedded, variegated, bioturbated carbonate mudstone to packstone formed. These facies represent platform deposition landward of the shoal environment in an open-circulation shallow lagoon.

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